1. Field of the Invention
This invention relates in general to electroplating hard metal on steel surfaces, and in particular to a method for electroplating a helical progressing cavity pump rotor.
2. Description of the Prior Art
A progressing cavity pump is a well known pump of a type that has an elastomeric outer element or stator. The stator has a double lead helix in its inner surface. A metal rotor having a single lead helical exterior inserts into the stator. When the rotor is rotated, discrete cavities are created between the single and double helixes, which progress to the discharge. The discrete cavities are sealed by a design interference fit between the very hard metal rotor and the relatively soft elastomer of the stator. Typically, the rotor is hardened steel, about 35 to 60 Rockwell "C", plated with up to 0.015 inches of hard chrome.
In electroplating systems in general, a DC power supply is utilized to impress a current flow through a solution of ionized plating material, so that these ions flow to the workpiece and plate out. Metal ions are positively charged, so that in chrome plating, the workpiece is a negatively charged pole, and positive electrodes are positioned adjacent the workpiece to establish the electrical current flow through the electrolytic bath.
Normally, to plate a helical rotor, positive pole electrodes are hung parallel to the rotor, and surrounding the rotor. This results in a difference in plating rate between the minor diameter and the major diameter portions of the helical rotor. The deposit will build up to a greater depth on the portions of the rotor that are closest to the electrodes.
In the prior art, the rotor is machined with the dimensions calculated to minimize the effects of the differential plating. The final product will have accurate dimensions, even though some of the plating will have greater thickness than other portions of the plated surface. In some areas, the chrome plating will be twice the depth than in other areas, requiring more plating thickness than is necessary. The additional plating thickness is expensive. Also, uneven wear will result.